Support WordReference API 'Entries' response

* Sometimes the WordReference API returns a response that includes
  Entries" instead of "PrincipalTranslations". This commit introduces a
  private method on ResponseParser, #translations_key, which enables
  support for 'Entries' if the key is present in the response.
* Issue documentation:
  https://github.com/malandrina/word_reference/issues/2